Transcription
Major Styles of Mineralization andHydrothermal Alteration and RelatedSolid- and Aqueous-Phase GeochemicalSignaturesBy Dana J. Bove, M. Alisa Mast, J. Bradley Dalton, Winfield G. Wright, andDouglas B. YagerChapter E3 ofIntegrated Investigations of Environmental Effects of HistoricalMining in the Animas River Watershed, San Juan County, ColoradoEdited by Stanley E. Church, Paul von Guerard, and Susan E. FingerProfessional Paper 1651U.S. Department of the InteriorU.S. Geological Survey
ContentsAbstract .165Introduction.165Methods.166Outline of Major Styles and Ages of Mineralization and Hydrothermal Alterationwithin the Animas River Watershed Study Area.168Pre-Ore Regional Propylitic Alteration Assemblage .169CEC Group .169CIC Group .170Geochemistry of Propylitically Altered Rocks .173Background Surface Water Chemistry .17826–25 Ma Molybdenum–Copper Porphyry Mineralization and Alteration .178Mount Moly Area Ore Deposits .178Hydrothermal Alteration in the Mount Moly Area .179Alteration Zones Near the Southeast Margin of the Silverton Caldera .181Geochemistry of Altered Rocks .181Mine Dump Compositions .182Mine Water Chemistry .182Background Surface Water Chemistry .18423 Ma Acid-Sulfate Alteration and Mineralization .192Red Mountains Area Ore Deposits .197Ohio Peak–Anvil Mountain Mineralization and Ore Deposits .199Hydrothermal Alteration .199Acid-Sulfate Alteration Assemblages .201Silicification .201Quartz-Alunite Alteration .201Quartz-Pyrophyllite and Argillic Assemblages .203Hydrothermal Sericitic Assemblages .203Quartz-Sericite-Pyrite Assemblage .203Weak Sericite-Pyrite Assemblage .204Origin, Age, and Relation to Intrusions .204Geochemistry of Altered Rocks in the Red Mountains and Ohio Peak–AnvilMountain Areas .204Mine Dump Compositions .205Red Mountains Area .205Ohio Peak–Anvil Mountain Area .205Mine Water Chemistry .206Red Mountains Area .206Ohio Peak–Anvil Mountain Area .206Background Surface Water Chemistry .207Red Mountains Area .207Ohio Peak–Anvil Mountain Area .208
18–10 Ma Polymetallic Vein Systems—Northeast-Trending Veins.208Eureka Graben Area .208Eureka Gulch, Treasure Mountain, and Placer Gulch Gold-Rich Veins .209Base-Metal Veins with Late-Stage Manganese Silicates .211Mineral Point .211California Gulch .211Zoning of Manganese Silicate Gangue Material .212Vein-Related Hydrothermal Alteration .212Rhyolites and Associated Mineralization .212Geochemistry of Altered Rock and Vein-Adjacent Zones .214Mine Dump Compositions .214Mine Water Chemistry .214Background Surface Water Chemistry .215Aqueous Geochemical Signatures of High-Silica Rhyolite Intrusionsand Late-Stage Vein Assemblages (F, W, Mo) .215 18 Ma Polymetallic Vein System—Northwest- and Northerly-Trending Veins .217South Silverton Area .217Western Shear System .219Eastern Shear System .219Vein-Related Hydrothermal Alteration .220Geochemistry of Altered Rocks .220Mine Dump Compositions .220Mine Water Chemistry .221Background Surface Water Chemistry .221Discussion and Summary .222Mount Moly Mineralization and Hydrothermal Alteration .222Red Mountains and OPAM Acid-Sulfate Areas .222Eureka Graben Area .224South Silverton Area .224Acid-Neutralizing Potential (ANP) of Minerals and Relationship to AlterationAssemblages .225Selected References .226Figures1. Map showing five areas that represent major styles and ages of mineralizationand alteration in the study area .1672. AVIRIS image of propylitic assemblage minerals in Boulder Gulch–Howardsvillearea .1713. AVIRIS images showing propylitic assemblage minerals west of Mineral Creek,in Niagara Gulch area, and in Kendall Mountain area .1724. Box plots of outcrop rock chemistry data from alteration assemblages in MountMoly, combined Red Mountains–OPAM, and Eureka Graben areas .1785. Alteration map of the Mount Moly alteration area .180
6. Box plots comparing surface and subsurface rock chemistry data from acidsulfate and quartz-sericite-pyrite zones in Red Mountains–OPAM and MountMoly alteration areas .1817. Box plots comparing mine dump data from the five mineralized areas .1848. Box plots comparing chemistry of all mine water samples to only thosewith pH 4.5 .1859. Graphs of metals versus pH for all mine waters from the five mineralized areas .18810. Box plots comparing low-pH mine water data from the five mineralized areas.19211. Box plots comparing geochemistry of background waters draining differentalteration assemblages .19612. Graph of dissolved sulfate versus calcium plus strontium in propylitic waterin Mount Moly alteration area .19713. Map showing zone of economically important breccia-hosted ore depositsin Red Mountains area.19814. Alteration map of Ohio Peak–Anvil Mountain (OPAM) area .20015. Alteration map of Red Mountains area .20216. Photograph showing highly silicified quartz-alunite-pyrophyllite breccia zonealong near-horizontal plane .20317. Generalized geologic and alteration map of Eureka Graben area .20918. Diagram showing paragenetic stages of vein mineralization from Eureka Grabenarea and adjacent areas .21019. Map of Eureka Graben area showing manganese silicate zoning trends .21320. Map showing fluoride data from mainstem streams and miscellaneous inflowssuperimposed on generalized alteration map of study area .21621. Map of South Silverton mineralized area .21822. Graph showing mean values of dissolved magnesium versus pH in backgroundwaters from the five mineralized areas .225Tables1. Point count data in volume percent for propylitic samples .1692. Analytical data for various mineral species.1703. Summary of rock geochemical data from the Mount Moly, Red Mountains,OPAM, and Eureka Graben areas .1754. Summary of mine dump geochemistry from five different mineralized areas.1835. Summary of mine water geochemistry from five different mineralized areas .1866. Ranges and geometric means of mine
Box plots comparing surface and subsurface rock chemistry data from acid-sulfate and quartz-sericite-pyrite zones in Red Mountains–OPAM and Mount Moly alteration areas .181 7. Box plots comparing mine dump data from the five mineralized areas .184 8. Box plots comparing chemistry of all mine water samples to only those with pH 4.5 .185 9. Graphs of metals versus pH for all mine .
